Secure domestic energy supply, global warming and climate change are presently receiving significant scientific, business, regulatory, political, and media attention. According to increasing numbers of independent scientific reports, greenhouse gases impact the ozone layer and the complex atmospheric processes that re-radiate thermal energy into space, which in turn leads to global warming on Earth. Warmer temperatures in turn affect the entire ecosystem via numerous complex interactions that are not always well understood. Greenhouse gases include carbon dioxide, but also include other gases such as methane, which is about 23 times more potent than carbon dioxide as a greenhouse gas, and nitrous oxide, which is over 300 times more potent than carbon dioxide as a greenhouse gas.
In addition to the foregoing greenhouse gas concerns, there are significant concerns about secure domestic energy supplies, concerns that the United States imports over 60% of the crude oil it consumes from a few unstable regions of the globe, and concerns about the rate at which global oil reserves are being depleted. Accordingly, there is an increasing focus on finding alternative sources of energy, including renewable, less expensive, and domestic energy sources that are cleaner to produce and use. These sources include coal seam methane, coal mine gas, non-conventional gas from shale deposits, and stranded well gas. These energy sources also include the organic fractions of municipal solid waste, food processing wastes, animal wastes, restaurant wastes, agricultural wastes, and waste water treatment plant sludge.
Many of the foregoing organic waste streams can be converted to biogas via anaerobic bacteria to produce mixtures of methane. Examples include covered landfills where the landfill gas contains approximately 48% methane, 38% carbon dioxide, 12% nitrogen and oxygen, water vapor, and small amounts of numerous other compounds. Biogas from anaerobic digestion of organic waste streams consists of approximately 65% methane, 33% carbon dioxide, water vapor and small amounts of other compounds. Coal mine gas contains approximately 64% methane, 32% nitrogen, 3% carbon dioxide, water, and small amounts of other compounds. Non-conventional or shale gas contains approximately 90% methane, 8% ethane and propane, 2% carbon dioxide, water, and small amounts of other compounds. Stranded well gas has a wide range of compositions depending on the location but typically contains approximately 80% methane, 13% nitrogen, several percent ethane and propane, plus water and 2% carbon dioxide. These stranded or waste sources of methane are widely geographically distributed rather than in large, localized sources like a large gas field. With enhanced technology these distributed methane sources are being converted to liquid natural gas (LNG) for effective storage, transport, and distribution to industrial end users for more economical use of process heat fuel and transportation end users for economical and low emissions vehicle fuel for light and heavy duty vehicles.
The processes associated with producing both LNG and compressed natural gas from LNG (LCNG) include purifying the incoming methane gas stream to remove constituents such as those that freeze out in or otherwise degrade LNG process equipment. Among the well known purification techniques is selective adsorption of certain impurities on different adsorbents such as activated alumina or zeolites. In such adsorption techniques certain impurities in a process stream flowing within a vessel containing the adsorbent are physi-adsorbed onto the surface of the adsorbent thus removing the impurities from the process stream. This purification continues until the adsorbent is saturated. To continue purification of the process stream, the process stream must be switched to another identical vessel containing clean, cool adsorbent. This transfer between vessels is normally accomplished by opening and/or closing a combination of several valves to accomplish a semi-continuous purification of the process stream. In one type of adsorption purifier, the saturated adsorbent is heated by several hundred degrees Fahrenheit, e.g., to ˜500° F., to substantially decrease the selective adsorptivity of the adsorbent. This heating thereby releases the impurities from the adsorbent so they can be purged from the vessel into a discharge stream before the clean adsorbent is cooled and prepared for another purification step. The heating, purging, and cooling steps accomplish regeneration of the adsorbent. This common type of adsorption purification is called temperature swing adsorption. It commonly involves two or more vessels in parallel which are interconnected by a complex set of control valves on the inlets and outlets of each vessel. The four stages of a temperature swing adsorption purification cycle are sequentially executed in each vessel nominally over a minimum period of several hours, e.g., 12 hours. One good application of temperature swing adsorption purification is to remove the carbon dioxide present in the methane mixtures from most of the distributed waste or stranded sources. The carbon dioxide must be efficiently removed to a concentration of about 100 parts per million to avoid freezing out in the cryogenic heat exchanger, a core component of a plant that produces LNG. The small, distributed nature of methane mixtures from many stranded gas wells, biomass waste streams or landfills makes the capital and operating costs associated with such unmonetized gas-to-LNG plants a key contribution to the delivered price of industrial process LNG fuel or LNG or LCNG vehicle fuel. Accordingly, there is a need for better purifier technology that simplifies the purification steps, provides continuous purification with fewer components such as valves, and thereby reduces capital and operating costs of such LNG plants and results in a less expensive methane fuel.